Systems and methods for space-time determinations with reduced network traffic

ABSTRACT

Space-time solutions are determined by exchanging pings among nodes in a network. Each ping includes a current space-time state of the transmitting node, which includes the transmitting node&#39;s currently estimated location and corrected time (as a count stamp). A particular node in the network receives pings from the other nodes in the network and uses the data in the received pings to estimate its own current position and to correct its own free-running clock relative to a common system time. As a service to the network, the particular node then transmits its corrected time (as a count stamp) and estimated position to the other nodes. In some embodiments, the space-time solutions discussed herein are used as backup to other navigation systems, such as the Automatic Dependent Surveillance-Broadcast (ADS-B) system.

TECHNICAL FIELD

This disclosure is related to object positioning systems. Moreparticularly, this disclosure is related to a single network node thatdetermines its own space-time solution based on data received from othernetwork nodes with which the single node communicates.

BACKGROUND INFORMATION

Current techniques for positioning include the following:

-   -   Cellular: Global Positioning System (GPS), Assisted GPS (A-GPS),        Time of Arrival (TOA), Radio Fingerprinting, Cell Global        Identity, and Enhanced Cell ID.    -   WiFi: Radio Fingerprinting, WiFi TOA, and Wireless Access Point        Wardriving.    -   GPS: Differential GPS (DGPS), Wide Area Augmentation System        (WAAS), A-GPS, and Laser Augmentation.

GPS is a space-based satellite constellation that provides constant,global geopositioning to end users. A user's GPS device processessignals from three or more GPS satellites, and by way of trilaterationprovides latitude and longitude data. GPS may be global, but it is notubiquitous because foliage, building edifices, and bridges block GPSsignals. The absence of GPS signals in places such as the urban core isproblematic because high value applications for reliable positioningabound, yet no reliable solution exists. The urban core in major citiesis also known as the “urban canyon,” because skyscrapers on both sidesof the streets block GPS signals. Another common problem is multipathdistortion in which signals bounce off many surfaces before reaching theuser, resulting in vastly inaccurate positioning data.

In addition to providing navigation within the urban core, GPS is widelyused to aid navigation worldwide. For example, GPS is a routinecomponent of aviation navigation at all levels including the nextgeneration air traffic control system referred to as Automatic DependentSurveillance-Broadcast (ADS-B). See,<<http://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=8145>> (lastaccessed Sep. 25, 2008). ADS-B is anchored on GPS derived position andvelocity information. ADS-B takes advantage of GPS accuracy, reliabilityand global coverage to improve navigation performance for improvedsafety and efficiency. Because of satellite-based navigation (SATNAV)vulnerabilities, however, it is widely recognized that ADS-B must have anetworked system-wide backup.

GPS was originally designed as a military system and has had recentbroad adoption for civil and civilian uses due to its inherent accuracyand a strong history of reliable performance. Several other entities,including the European Union, Russia, India, China, and Japan havesatellite-based Position, Navigation, and Timing. Collectively, the useof satellite-based navigation services is referred to a GlobalNavigation Satellite Systems (GNSS). GPS, however, remains the de factosystem of choice, primarily because of its maturity, with over fifteenyears of reliable free service. GPS acceptance has also been enhanced byopen interface control documentation (ICD), which allows receivermanufacturers to confidently design systems against a reliable standard.

These satellite systems provide a very low cost-of-ownership for manyapplications including, for example, civil aviation management comparedto traditional radar methods and have the added advantage of globalcoverage for international flights. However, because they aresatellite-based, there is a danger that either natural or human threatsto these systems could make them vulnerable to outages. Thus, there isconsiderable work going into a search for an aviation back-up system toprovide navigation information in the event of GPS outage. See, forexample,<<http://www.jpdo.gov/library/InformationPapers/ADS-B_Backup_Report_PDA.pdf>>(last accessed Sep. 26, 2008).

SUMMARY OF THE DISCLOSURE

Space-time solutions are determined by exchanging ping transmit eventsamong nodes in a network. Each ping transmit event includes a currentspace-time state of the transmitting node, which includes thetransmitting node's currently estimated location and corrected time (asa count stamp). A particular node in the network receives ping transmitevents from the other nodes in the network and uses the data in thereceived ping transmit events to estimate its own current position andto correct its own free-running clock relative to a common system time.As a service to the network, the particular node then transmits itscorrected time (as a count stamp) and estimated position to the othernodes.

In one embodiment, a method of determining location and timinginformation of a local node in a network includes receiving pingtransmit events from a plurality of nodes in the network. Each pingtransmit event includes a transmit count stamp corresponding to a remotetime (based on an independent remote clock) when the respective nodetransmits the ping transmit event, and a remote location of therespective node at the remote time when the respective node transmitsthe ping transmit event. For each ping transmit event received within apredetermined number of local clock counts (based on an independentlocal clock), the method also includes associating a receive count stampwith the received ping. The method further includes processing thetransmit count stamps, the remote locations, the receive count stamps,and the local locations corresponding to the ping transmit eventsreceived within a predetermined number of local clock counts todetermine one or more clock solutions for correlating the independentlocal clock with a network time, and to determine one or more locationsolutions for estimating a new local location.

In another embodiment, a system for determining location and timinginformation includes a plurality of nodes in communication with oneanother. Each node includes an independent clock for generating countstamps, a communication device for receiving remote ping transmit eventsfrom the other nodes, and a processor. The processor is configured toassociate each remote ping transmit event with a receive count stampbased on the independent clock. The processor is also configured toprocess the transmit count stamps, the remote locations, the receivecount stamps, and the local locations to determine one or more clocksolutions for correlating the independent clock with a network time, andto determine one or more location solutions for estimating a new locallocation.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a network of nodes according to one embodiment.

FIG. 2 is a block diagram of a node according to one embodiment.

FIG. 3 is a graph showing clock counts corresponding to twotime-displaced ping transmit events produced by a first node andreceived by a second node under conditions in which the nodes are thesame distance apart from each other during the two ping transmit events.

FIG. 4 is a graph showing clock counts corresponding to twotime-displaced ping transmit events produced by a first node andreceived by a second node under conditions in which the nodes aredifferent distances apart from each other during the two ping transmitevents.

FIG. 5 is a graph showing clock counts corresponding to twotime-displaced ping transmit events produced by a first node andreceived by a second node under conditions in which the nodes are thesame distance apart from each other during the two ping transmit eventsbut their clock rates are dissimilar.

FIG. 6 is a diagram for use in illustrating the calculation of a changein distance between the first and second nodes.

FIG. 7 is a diagram showing two timelines of ping transmit eventsproduced by the first node and received by the second node forcalculating changes in rates of the clocks of the first and secondnodes.

FIG. 8 is a schematic diagram of a network that includes at least twonodes according to one embodiment.

FIG. 9 is a schematic diagram illustrating example relationships betweena first node's counter and a ping transmit event received from a secondnode according to one embodiment.

FIG. 10 is a flowchart of a method for determining location and timinginformation of a local node in a network according to one embodiment.

FIG. 11 schematically illustrates nodes in an Automatic DependentSurveillance-Broadcast (ADS-B) system according to one embodiment.

FIG. 12 illustrates a table that compares limitations and features ofvarious SATNAV backup systems under consideration for aviation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A precision timing/location technology referred to as “PhaseNet” isdescribed in International Patent Application Publication No.WO/2008/073347, filed Dec. 7, 2007, by Geoffrey Rhoads (the “Rhoadsapplication”). As discussed in detail below, certain applications of thePhaseNet technology exchange a large amount of information between nodesin a network in order for a particular node to determine its ownspace-time solution. In some applications of the PhaseNet technology,the amount of data required to be exchanged between nodes may beundesirable because it overly congests the network or exceeds thenetwork's ability to timely communicate the data. Thus, certainembodiments disclosed herein provide an enhanced PhaseNet technologythat substantially reduces the amount of data exchanged between nodesfor space-time calculations.

In one embodiment disclosed herein, each node in a network transmitstime stamped messages (ping transmit events) to other nodes in thenetwork. Each ping transmit event includes a current space-time state ofthe transmitting node. The current space-time state includes thetransmitting node's currently estimated location and corrected time (inthe form of a transmit count stamp). A particular node P in the networkproduces ping receive events by receiving and associating receive countstamps to the ping transmit events received from the other nodes in thenetwork. Node P uses the data in the received ping transmit events toestimate node P's current position and to correct node P's free-runningclock relative to a common network system time. In one embodiment, thecommon network system time is defined as an average of the node clockswithin the system. Artisans will recognize from the disclosure hereinthat many other system clock determination techniques are possible. Forexample, if one the system nodes operates with an atomic clock, thatclock can establish the system clock with extremely small clock drift.

As discussed in detail below, ping transmit events received from aminimum number of other nodes may be required in certain embodiments inorder for node P to derive a solution for correcting its currentspace-time state. As a service back to the network, node P in turnbroadcasts its corrected space-time state (the newly estimated locationand corrected time in the form of a count stamp) to the network as aping transmit event for use by the other nodes in determining their ownrespective space-time solutions.

In certain embodiments, each node in the network includes an initialspace-time state that provides a starting point for sending pingtransmit events to the other nodes and for determining a solution forcorrecting its own space-time state. The initial space-time states maybe provided using other timing/location systems and methods. Forexample, the initial space-times state may be provided by a GPS systemor by using the PhaseNet technology described in the Rhoads application.As another example, a user may manually enter the initial space-timestate into a node based on known methods for calculating time andlocation data based on observation. The initial state information mayalso include initial direction vectors between network nodes, asdiscussed below.

In certain embodiments, the enhanced PhaseNet technology disclosedherein is used as a backup to other timing/location systems that providethe initial space-time state to the nodes. In one example aviationembodiment discussed below, the enhanced PhaseNet technology disclosedherein is used as backup for an Automatic DependentSurveillance-Broadcast (ADS-B) system to reduce vulnerability to GPSoutages. During normal operation, for example, the ADS-B system uses GPSto provide timing and location information to aircraft. When GPS becomesunavailable, however, each ADS-B node (e.g., aircraft, airport controltowers, communication stations, fixed beacons, and/or other devices incommunication with the aircraft) determines its initial space-time statebased on its last known time and location (e.g., as provided by the GPSbefore the outage). Each ADS-B node transmits its initial space-timestate as a ping transmit event and receives ping transmit events fromother ADS-B nodes. Each ADS-B node then uses its initial space-timestate and the space-time states in the ping transmit events receivedfrom the other ADS-B nodes to update its own space-time solution. Thus,the embodiments disclosed herein allow ADS-B nodes to “ride through”periods of GPS outage with minimal degradation of system accuracy.

Reference is now made to the figures in which like reference numeralsrefer to like elements. For clarity, the first digit of a referencenumeral indicates the figure number in which the corresponding elementis first used. In the following description, numerous specific detailsare provided for a thorough understanding of the embodiments disclosedherein. However, those skilled in the art will recognize that theembodiments described herein can be practiced without one or more of thespecific details, or with other methods, components, or materials.Further, in some cases, well-known structures, materials, or operationsare not shown or described in detail in order to avoid obscuring aspectsof the embodiments. Furthermore, the described features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

General PhaseNet Overview

In general, the PhaseNet algorithms (discussed below) run on the nodesof a network, and messages passed between the nodes serve as input tothe algorithms. FIG. 1 illustrates a network that includes ten nodes A,B, C, D, E, F, G, H, I, J according to one embodiment. The network 100may include different numbers of nodes and nodes may be added to orremoved from the network 100 at any time. In this example, it is assumedthat some of the nodes move with respect to the other nodes. Forexample, some of the nodes may be located in moving vehicles such asautomobiles or airplanes. Lines between nodes represent communicationlinks (either a duplex link or a monoplex link). For simplicity, FIG. 1does not show lines between each node. However, it is assumed that eachnode may establish duplex communication with any of the other nodes.Further, while the nodes in FIG. 1 are illustrated as being located in atwo-dimensional plane, an artisan will recognize from the disclosureherein that nodes may be distributed in a three-dimensional space.

FIG. 2 is a block diagram of a node 200 according to one embodiment. Thenode 200 shown in FIG. 2 may correspond, for example, to one or more ofthe nodes A, B, C, D, E, F, G, H, I, J shown in FIG. 1. The node 200includes a processor 210 in communication with a memory device 212, acounter 214, and a communication device 216. The processor 210 mayinclude, for example, digital signal processors, one or morefield-programmable gate array (FPGA), general purpose single-chip ormulti-chip microprocessors, special purpose processors, combinations ofthe foregoing, or the like. The memory device 212 may include, forexample, random access memory (RAM), hard drives, drives that accepthard or floppy disks, tape cassettes, CD-ROM, DVD-ROM, or othercomputer-readable storage media. The memory device 212 includes programinstructions that are executable by the processor 210 for determiningspace-time solutions as described herein. The memory device 212 may alsoinclude one or more databases (not shown) for storing data used tocalculate the space-time solutions.

The communication device 216 is configured to provide communication withother nodes. In certain embodiments, the communication device 216 alsoprovides an interface to other timing/location systems such as a GPSdevice. The communication device 216 may provide access to a variety ofdifferent types of networks that proliferate throughout our environmentsuch as WiMax, WiFi, Cellular, Mesh Networks, Ad Hoc Networks, FirstResponder Radio, and Spaceborne systems such as Iridium and the GPSconstellation itself. The communication device 216 may also provideaccess to wireline networks such as wireline IT networks,synchronization, geodetic remote sensing, and fiber optic backbonecalibration networks. An artisan will recognize from the disclosureherein that many different communication networks and/or protocols maybe used depending on the particular application. In the example ADS-Bembodiment discussed below, for example, the communication device 216uses the existing media access control (MAC) layer of the universalaccess transceiver (UAT) for communication between ADS-B nodes.

In one embodiment, the counter 214 is driven by a low cost digital clock(not shown). The counter 214 may have at least a 64 bit counting range.In one embodiment, the counter 214 is capable of running atapproximately 1 million counts per second. The counter 214 may be builtusing cascades of counters, with 8 or 16 bit counters running at thehighest speed, and driving lower rate 64 bit counters, for example. Ofcourse, an artisan will recognize from the disclosure herein that manyother configurations may also be used. Given that PhaseNet solutionsultimately solve for count-rate variabilities between nodes, theirquality may be commensurate with extremely low-cost parts and very basicperformance specifications.

The PhaseNet algorithms weave positioning and timing services intowireless communication networks, and may complement other location andtiming technologies or function independently. The algorithms may bedeployed across a wide variety of physical and network layer protocols.The PhaseNet algorithms exploit at least two aspects of a wirelessnetwork. The first aspect is that, by nature, the nodes in a wirelessnetwork are configured to pass messages between one another. The secondaspect of a wireless network used by the PhaseNet algorithms is thateach node has a local clock that may be used to time stamp the sendingand receiving of messages.

In the general case, the messages passed between two network nodes takethe form of pings and pungs. A ping transmit event includes a timestamped message from one node to another. The sending node appends tothe message the value of its counter at the instant the message istransmitted. The receiving node then takes note of the value of its owncounter when the message is received. The data resulting from a pingevent includes, most basically, of a pair of count values. The firstcount is the clock value of the sending node when the ping transmitevent was sent, and the second count is the clock value of a receivingnode when it received the ping transmit event. The term pung is used torefer to any data communication between nodes of the network, such assharing the data resulting from ping events, which is not itself a pingevent. The general PhaseNet algorithms take the data resulting from pingevents and pungs and use it to solve for timing and/or locationinformation useful for the nodes of the network. The specific form ofthis information is application dependent.

The method used by the PhaseNet algorithms to determine the locations ofnodes is pseudo lateration. In lateration, the distances (or differencesof distances) between nodes, derived by time-of-flight measurements, areused to solve for positions of nodes. However, the PhaseNet algorithmsinvoke lateration as but one element of a more sophisticated structure.This is because PhaseNet synthesizes both timing and positioninformation for each node of the network. These two elements, timing andposition, are intertwined.

To determine the positions of the nodes of a network using purelateration implies accurate timing information is available to maketime-of-flight measurements. Conversely, to synchronize the clockswithin a network by passing synchronization messages between nodesrequires that node positions are known, so that time-of-flight delaysmay be subtracted out. The PhaseNet algorithms handle the linked natureof time and space by solving for both elements simultaneously. Thisallows Phase-Net to do the following:

-   -   Propagate position information throughout a network, even when        only a few nodes have GNSS capability and/or access.    -   Enhance the accuracy of Inertial Navigation System (INS) by        developing a network solution using INS sensors across the        network.    -   Provide a background capability that is always available as a        GNSS backup.

In the most general case, the PhaseNet algorithms start withfree-running clocks on each network node and, from these, synthesize acommon network time and a relative location solution for the network.Before the PhaseNet algorithms are run, there may not be pre-existingtiming relationship between nodes, and no concept of what a “networktime” might be. The free-running counters are used to time stampmessages passed between nodes, and the resulting time stamps are thenprocessed by the algorithms.

Example Embodiment Using Pung Messages

The following description presents a specific implementation of thecomputations performed and solutions generated to achieve overall systemoperation using pung messages according to one embodiment.

FIG. 3 is a graph showing clock counts corresponding to twotime-displaced ping events transmitted from node A and received by nodeB under conditions in which nodes A and B are the same distance apartfrom each other during two ping events. With reference to FIG. 3, avector PE1 represents a first ping event, which has a clock count value,Ping 1, expressed as

Ping 1=C _(r1B) −C _(t1A),   (1)

where C_(t1A) is a clock count (or count stamp) accumulated by a counter(e.g., counter 214 shown in FIG. 2) driven by a digital clock residingat node A and transmitted by node A at a time, T_(t1A), and C_(r1B) is aclock count (or count stamp) accumulated by a counter driven by adigital clock residing at node B and associated with a time, T_(r1B), atwhich node B receives the first ping transmit event produced by node Aat T_(t1A). A vector PE2 represents a second, later ping event, whichhas a clock count value, Ping 2, expressed as

Ping 2=C _(r2B) −C _(t2A),   (2)

where C_(t2A) is the clock count transmitted by node A at a time,T_(t2A), and C_(r2B) is the clock count associated with a time, T_(r2B),at which node B receives the second ping transmit event produced by nodeA at T_(t2A). The straight line (ignoring incremental countquantization) plot of clock counts as a function of time for each ofnodes A and B indicates that their respective digital clocks, CLK_(A)and CLK_(B), operate at the same or a “system nominal” rate.

A differential clock count value representing the difference betweenPing 2 and Ping 1, ΔPing_(AB), can be expressed as

$\begin{matrix}{{\Delta \; {Ping}_{AB}} = {{{{Ping}\; 2} - {{Ping}\; 1}} = {( {C_{r\; 2B} - C_{t\; 2A}} ) - {( {C_{r\; 1B} - C_{t\; 1A}} ).}}}} & (3)\end{matrix}$

The entity ΔPing_(AB)=0 when nodes A and B are the same distance apartfrom (i.e., not moving relative to) each other at the times of pingevents PE1 and PE2. This is the situation represented in FIG. 3, inwhich (T_(r1B)−T_(t1A)) and (T_(r2B)−T_(t2A)) are equal.

FIG. 4 is a graph showing clock counts corresponding to twotime-displaced ping events transmitted from node A and received by nodeB under conditions in which nodes A and B are different distances apartfrom each other during the two ping events. With reference to FIG. 4, avector PE1′ represents a first ping event having a Ping 1′ value that isthe same as the Ping 1 value of vector PE1. A vector PE2′ represents asecond, later ping event having a Ping 2′ value that is greater than thePing 2 value of vector PE2. A change in distance between nodes A and Bfor the first and second ping events is expressed as ΔDist_(AB). Theinequalities ΔPing_(AB)>0 and ΔDist_(AB)>0 indicate that nodes A and Bmoved farther apart from each other between the times of the first andsecond ping events, as represented in FIG. 4. Similarly, theinequalities ΔPing_(AB)<0 and ΔDist_(AB)<0 indicate that nodes A and Bmoved closer to each other between the times of the first and secondping events.

FIG. 5 is a graph showing clock counts corresponding to twotime-displaced ping events transmitted from node A and received by nodeB under conditions in which nodes A and B are the same distance apartfrom each other during the two ping events but their clock rates aredissimilar. With reference to FIG. 5, (T_(r1B)−T_(t1A)) and(T_(r2B)−T_(t2A)) are equal; therefore, nodes A and B are not movingrelative to each other at the times of the first and second ping eventsPE1″ and PE2″. The clock count plots of nodes A and B indicate that theyare not parallel and that the node A clock, CLK_(A), counts at a slowerrate than the count rate of the node B clock, CLK_(B). FIG. 5 indicatesthat when the clock rate of CLK_(A) decreases relative to the systemnominal rate, Ping 2″ increases relative to Ping 2 of FIG. 3. Ingeneral, the following relationships characterize in ping counts changesin rate of node clock A, ΔCLK_(A), and node clock B, ΔCLK_(B):

-   -   ΔCLK_(A) decreases        ΔPing_(AB) increases    -   ΔCLK_(B) decreases        ΔPing_(AB) decreases.

The following two equations express, in terms of ping counts, changes inthe distance between nodes A and B, assuming that ping events are alsotransmitted from node B and received and count stamped by node A:

ΔPing_(AB) =K ₁ΔDist_(AB) −K ₂ΔCLK_(A) +K ₃ΔCLK_(B)   (4)

ΔPing_(BA) =K ₁ΔDist_(BA) +K ₂ΔCLK_(B) −K ₃ΔCLK_(A)   (5)

FIG. 6 is a diagram for use in illustrating the calculation ofΔDist_(AB). For small displacements during a unit ping interval (i.e.,during a short interval between successive pings), the term

ΔDist_(AB)=√{square root over (ΔX _(AB) ² +ΔY _(AB) ² +ΔZ _(AB) ²)}  (6)

can be approximated. FIG. 6 is a diagram showing a straight line pathsegment between nodes A and B. With reference to FIG. 6, a straight line610 connecting nodes A and B represents in x, y coordinate space thedisplacement of node B relative to node A for two successive pingevents. FIG. 6 shows that, for short time intervals between successiveping events and when node A remains stationary, the x and y componentsof ΔDist_(AB) at node B can be expressed as ΔB_(x) cos θ and ΔB_(y) cosα, respectively, where ΔB_(x) and ΔB_(y) are the changes in therespective x and y coordinates of node B from its receipt of Ping 1 toits receipt of Ping 2, θ is the angle between line 610 and itsprojection onto the x axis, and α is the angle between line 610 and itsprojection onto the y axis. Similarly, in x, y, z coordinate space, thez component of ΔDist_(AB) can be expressed as ΔB_(z) cos φ.

When the three components are combined and the coordinates of node A areincluded, ΔDist_(AB) can be expressed as

$\begin{matrix}{{\Delta \; {Dist}_{AB}} = {{\Delta \; B_{x}\cos \; \theta} + {\Delta \; B_{y}\cos \; \alpha} + {\Delta \; B_{z}\cos \; \phi} - {\lbrack {{\Delta \; A_{x}\cos \; \theta} + {\Delta \; A_{y}\cos \; \alpha} + {\Delta \; A_{z}\cos \; \phi}} \rbrack.}}} & (7)\end{matrix}$

Substituting into equation (4) the expression for ΔDist_(AB) in equation(7) and taking into account the speed of light, c, for the E-Mimplementation provides

$\begin{matrix}{{\Delta \; {Ping}_{AB}} = {{\Delta \; {B_{x}( \frac{\cos \; \theta}{c} )}} + {\Delta \; {B_{y}( \frac{\cos \; \alpha}{c} )}} + {\Delta \; {B_{z}( \frac{\cos \; \phi}{c} )}} + {\Delta \; {A_{x}( \frac{{- \cos}\; \theta}{c} )}} + {\Delta \; {A_{y}( \frac{{- \cos}\; \alpha}{c} )}} + {\Delta \; {A_{z}( \frac{{- \cos}\; \phi}{c} )}} - {K_{2}\Delta \; {CLK}_{A}} + {K_{3}\Delta \; {{CLK}_{B}.}}}} & (8)\end{matrix}$

Simplifying equation (8) by relabeling the constant coefficients of theterms of ΔDist_(AB),

ΔPing_(AB) =ΔA _(X) K _(AX) +ΔB _(X) K _(BX) +ΔA _(Y) K _(AY) +ΔB _(Y) K_(BY) +ΔA _(Z) K _(AZ) +ΔB _(Z) K _(BZ) −K ₂ ΔCLK _(A) +K ₃ ΔCLK _(B).  (9)

The solution of the ΔCLK_(A) and ΔCLK_(B) terms is developed withreference to FIG. 7. In FIG. 7, the horizontal line A represents atimeline of ping transmit events produced by node A, and the horizontalline B represents a timeline of the ping receive events produced by nodeB. The vertical lines intersecting horizontal lines A and B are mutuallyspaced apart by a unit time interval, which represents the period of thesystem nominal clock rate. The shorter-length tick marks on lines A andB indicate the actual clock rates of CLK_(A) and CLK_(B), respectively.A vector PE1 represents a first ping event transmitted by a node A at atime A_(t1) established by CLK_(A) and received by node B at a timeB_(r1) established by CLK_(B). A vector PE2 represents a second pingevent transmitted by node A at a later time A_(t2) established byCLK_(A) and received by node B at a time B_(r2) established by CLK_(B).Transmit times A_(t1) and A_(t2) define respective time points P₁ andP₂, and receive times B_(r1) and B_(r2) define respective time points P₃and P₄. Inspection of FIG. 7 reveals that

P ₁ P ₂ + P₂ P ₄ = P₁ P ₃ + P₃ P ₄ .   (10)

The term P₁P₂ represents the time interval, measured in system nominaltime, between the transmission of PE1 and the transmission of PE2.Similarly, the term P₃P₄ represents the system nominal time intervalbetween the reception times for these ping events. The terms P₂P₄ andP₁P₃ represent the system nominal time intervals between, respectively,the transmission and the reception of PE2 and PE1. More specifically,with reference to FIG. 3,

P ₁ P ₂ =(C _(t2A) −C _(t1A))−ΔCLK _(A12)   (11)

P ₃ P ₄ =(C _(r2B) −C _(r1B))−ΔCLK _(B12),   (12)

where ΔCLK_(A12) and ΔCLK_(B12) represent the number of clock ticksneeded to correct to the system nominal clock rate for, respectively,CLK_(A) from the transmission time of first ping event PE1 to thetransmission time of second ping event PE2 and for CLK_(B) from thereceive time of PE1 to the receive time of PE2. Moreover, with referenceto FIG. 7,

$\begin{matrix}{\overset{\_}{P_{1}P_{3}} = {\frac{{Dist}_{1{AB}}}{c}\mspace{14mu} {and}}} & (13) \\{{\overset{\_}{P_{2}P_{4}} = \frac{{Dist}_{2{AB}}}{c}},} & (14)\end{matrix}$

where Dist_(1AB) represents for the first ping event, PE1, the distancebetween nodes A and B from the transmit time recorded at node A to thereceive time recorded at node B, and Dist_(2AB) represents for thesecond ping event, PE2, the distance between nodes A and B from thetransmit time recorded at node A to the receive time recorded at node B.Thus, equation (6) also can be expressed as

$\begin{matrix}{\frac{\Delta \; {Dist}_{AB}}{c} = {\frac{{Dist}_{2{AB}}}{c} - {\frac{{Dist}_{1{AB}}}{c}.}}} & (15)\end{matrix}$

Substituting into equation (10) the right-hand side terms of equations(11), (12), (13), and (14) provides the following expression

$\begin{matrix}{{( {C_{t\; 2A} - C_{t\; 1A}} ) - {\Delta \; {CLK}_{A\; 12}} + \frac{{Dist}_{2{AB}}}{c}} = {\frac{{Dist}_{1\; {AB}}}{c} + ( {C_{r\; 2B} - C_{r\; 1B}} ) - {\Delta \; {{CLK}_{B\; 12}.}}}} & (16)\end{matrix}$

Rearranging the terms of equation (10) provides

$\begin{matrix}{{{{- \Delta}\; {CLK}_{A\; 12}} + \frac{{Dist}_{2\; {AB}}}{c}} = {\frac{{Dist}_{1\; {AB}}}{c} + ( {C_{r\; 2B} - C_{t\; 2A} - C_{r\; 1B} + C_{t\; 1A}} ) - {\Delta \; {{CLK}_{B\; 12}.}}}} & (17)\end{matrix}$

Substituting into equation (17) the left-hand side terms of equations(3) and (15) results in the following expression

$\begin{matrix}{{{\Delta \; {Ping}_{AB}} = {\frac{\Delta \; {Dist}_{AB}}{c} + {\Delta \; {CLK}_{B\; 12}} - {\Delta \; {CLK}_{A\; 12}}}},} & (18)\end{matrix}$

where ΔCLK_(B12) and ΔCLK_(A12) represent corrections to, respectively,CLK_(B) and CLK_(A) to comport with the system nominal clock rate.

Equation (18) is in the form of the equation to which matrix algebra isapplied to solve for the unknown displacement values and changes inclock rates. The matrix equation is expressed as

g=Hf,   (19)

where g is a column vector of ΔPings, the number of which is the numberof ping events minus 1; H is a two-dimensional matrix of coefficientsconstructed from the ping events; and f is a column vector of unknownsthat include changes in clock rate and location changes in x, y, and zdisplacements.

Referring again to the example of ten nodes A, B, C, D, E, F, G, H, I, Jshown in FIG. 1, each node listens to and records the other nine nodes'ping transmit events, yielding 9×10 or 90 ping events that are thenrecorded. Thus, in one embodiment, the g vector is organized in groupsof 90, corresponding to roughly synchronous ping events of the 45 duplexchannels existing among the ten nodes. In another embodiment, PhaseNetorganizes the g vector in short snippets of information of a size equalto the length of a “harmonic block.” In the example embodiment shown inFIG. 1, for example, this length is defined as 10 milliseconds ornominally 10 ping epochs for any given node. Many PhaseNetimplementations may choose between 5 and 100 fundamental ping epochs perharmonic block, being a trade-off between, on the one hand, flexibilityin dealing with different sampling rates on the chosen metrics, and onthe other hand, inverting very large matrices. As computing resourcescontinue to improve, the choice will slowly move beyond “100”, as thepressure to worry about the size of matrices and the speed of inversionslessens.

The computation of equation (19) is carried out using harmonic blocks,in which there is a selected number of harmonic blocks for each equationand selected numbers of clock solutions and location solutions for eachharmonic block. The number of system nodes can change (above a certainminimum number of nodes), depending on whether certain nodes remain inthe system.

Before equation (19) is solved, all of the ping information isaccumulated by at least one node in the network. Each node uses a pungbroadcasting schedule according to certain embodiments to transmit toother nodes in the network the ping information the node has received.By combining ping events and pungs that have been received, a node isable to reconstruct information for all of the ping events of thenetwork.

In the example embodiment shown in FIG. 1, an entire harmonic block'sworth of accumulated ping information is sent to a node's communicationdevice 216 (see FIG. 2) for broadcast as a pung packet to all othernodes. Nodes receiving this pung-packet message (from at least oneharmonic block time in the past) store the received pung data along withits own accumulated ping information. In this example, the pung datapacket includes nine other nodes' received ping data. This configurationin which all participating nodes share all information is a baselinesolution example, such that any node can create a full set-widesolution. Other embodiments may designate special nodes that capture allthe pung packet data and thus have the full information set necessary tocalculate set-wide solutions.

In the example embodiment shown in FIG. 1, a pung structure associatedwith one harmonic block should have 81 other-node ping information×10ping events per block×2 bytes per ping data point or 1,620 bytes ofinformation if no down-sampling or compression is used. This amounts to18,000 bytes of pung information per channel per second, which mayexceed the communication channel's capacity. In certain embodiments,only 3,600 bytes of accumulated ping data per second are stored, thuslimiting the data transfer rate to 10 Kbps. Thus, in certainembodiments, PhaseNet reduces “Pung Overhead,” the communications demandfor the pung channel, to only a small percentage of a given channel'sdata carrying capacity. At the very least, the pung data rate ispreferably below the channel capacity. Certain embodiments compress thepung data using standard algorithms known to those skilled in the art toreduce the burden on the communication channel.

Enhanced PhaseNet Algorithms with Reduced Data Flow

In addition to using compression, or in other embodiments, enhancedPhaseNet algorithms reduce the amount of data required to be exchangedbetween nodes for space-time calculations. In such embodiments, eachnode does not transmit a digest (pung) of all the ping events it hasheard throughout the network, as discussed in the embodiments above.Rather, each node produces a ping transmit event that includes its owncurrent space-time state. The current space-time state includes thetransmitting node's currently estimated location and corrected time (inthe form of a count stamp). Thus, the ping transmit events used in theenhanced PhaseNet algorithms discussed herein are different than theping transmit events used in the general PhaseNet algorithms discussedabove in that they also include the transmitting node's currentlyestimated location.

For example, FIG. 8 is a schematic diagram of a network 800 thatincludes at least two nodes, node A and node P, according to oneembodiment. Nodes A and P may each include, for example, the elements ofnode 200 discussed above in relation to FIG. 2. From the viewpoint ofnode P, node P receives a constant stream of ping transmit events fromnode A, each of which includes a count stamp C_(tA) corresponding to atime T_(tA) when node A transmitted the particular ping transmit event.Each ping transmit event also includes a location (A_(X), A_(Y), A_(Z))of node A at the transmit time T_(tA). As discussed in detail below,node P constructs one or more equations from one or more correspondingping transmit events received from node A, and combines the equationwith other equations constructed from other ping transmit eventsreceived from other nodes to determine clock solutions CS and locationsolutions LS. A clock solution CS indicates the number of clock countsto add to node P's (free-running) clock to account for clock drift withrespect to a system time, as discussed above. A location solution LSindicates the amount of correction (e.g., in X, Y, and Z directions)needed to estimate node P's position. As a service back to the network800, node P in turn broadcasts its corrected space-time state (the newlyestimated location and corrected time in the form of a count stamp) tothe network 800 as a ping transmit event for use by the other nodes(e.g., including node A) in determining their own respective space-timesolutions.

FIG. 9 is a schematic diagram illustrating example relationships betweennode P's counter and a ping transmit event received from node Aaccording to one embodiment. In this example, node P receives a pingtransmit event from node A at a receipt clock count C_(rPA) of 1400counts on node P's (uncorrected) counter. At intervals of 1000 clockcounts, measured on node P's free-running clock, node P calculates newlocation solutions LS1, LS2, and LS3 (e.g., illustrated at clock countsC_(LS1), C_(LS2), and C_(LS3)). At half this rate, node P calculates newclock solutions CS1, CS2 (e.g., illustrated at clock counts C_(CS1) andC_(CS2)).

In this example, it is assumed that the ping transmit event from node Aincludes the corrected clock count C_(tA) of node A at the transmit timeT_(tA), as well as the corrected location (A_(X), A_(Y), A_(Z)) of nodeA at the transmit time T_(tA). Thus, it may be assumed that there are noerrors with these quantities, as well as no error with the receipt clockcount C_(rPA) of 1400 counts. It may also be assumed for this examplethat node P begins with a reasonably accurate estimation of its ownlocation (P_(X), P_(Y), P_(Z)). Based on these assumptions, equation(20) below may be written in terms of the unknown clock solutions CS1,CS2 and the unknown location solutions LS1, LS2, LS3 for node P:

$\begin{matrix}{{{( {C_{rPA} - C_{tA}} ) - \frac{\sqrt{( {A_{X} - P_{X}} )^{2} + ( {A_{Y} - P_{Y}} )^{2} + ( {A_{Z} - P_{Z}} )^{2}}}{c}} = {{k_{X}\lbrack {{m_{A}( {{LS}\; 1_{X}} )} + {n_{A}( {{LS}\; 2_{X}} )}} \rbrack} + {k_{Y}\lbrack {{m_{A}( {{LS}\; 1_{Y}} )} + {n_{A}( {{LS}\; 2_{Y}} )}} \rbrack} + {k_{Z}\lbrack {{m_{A}( {{LS}\; 1_{Z}} )} + {n_{A}( {{LS}\; 2_{Z}} )}} \rbrack} - \lbrack {{q_{A}( {{CS}\; 1} )} + {r_{A}( {{CS}\; 2} )}} \rbrack}},} & (20)\end{matrix}$

where m_(A) and n_(A) are constants determined by the location solutionclock counts C_(LS1), C_(LS2) in relation to the receipt clock countC_(rPA), and q_(A) and r_(A) are constants determined by the clocksolution clock counts C_(CS1), C_(CS2) in relation to the receipt clockcount C_(rPA). In particular, m_(A)+n_(A)=1, q_(A)+r_(A)=1, and for theexample clock count values shown in FIG. 9:

$\begin{matrix}{{m_{A} = {\frac{C_{{LS}\; 2} - C_{rPA}}{C_{{LS}\; 2} - C_{{LS}\; 1}} = {\frac{2000 - 1400}{2000 - 1000} = 0.6}}},} & (21) \\{{n_{A} = {\frac{C_{rPA} - C_{{LS}\; 1}}{C_{{LS}\; 2} - C_{{LS}\; 1}} = {\frac{1400 - 1000}{2000 - 1000} = 0.4}}},} & (22) \\{{q_{A} = {\frac{C_{{CS}\; 2} - C_{rPA}}{C_{{CS}\; 2} - C_{{CS}\; 1}} = {\frac{3000 - 1400}{3000 - 1000} = 0.8}}},{and}} & (23) \\{r_{A} = {\frac{C_{rPA} - C_{{CS}\; 1}}{C_{{CS}\; 2} - C_{{CS}\; 1}} = {\frac{1400 - 1000}{3000 - 1000} = {0.2.}}}} & (24)\end{matrix}$

The entity k_(XYZ) _(—) _(AP)={k_(X), k_(Y), k_(Z)} may be referred toherein as the “coarse direction vector” existing between node A and nodeP, and the scalar components k_(X), k_(Y), and k_(Z) may be referred toherein as “direction cosines” divided by c (the speed of light) of thecoarse direction vector. The word “coarse” is used because strictdirection is only asymptotically defined, and yet direction can beutilized nevertheless. In its strictest form, the coarse directionvector is simply the starting estimate on a convergence sequence, butfor all practical purposes, a small percent error in the directionvectors is trivial compared to error analysis. One of the roles for thecoarse direction vectors is to establish a Cartesian coordinate systemsuch that motion can be resolved into orthogonal components that makesense to both the transmitting node and the receiving node (andeventually the entire set of nodes). As discussed above, initialdirection vectors may be used in some embodiments based on the lastknown relative positions of the sending and receiving nodes.

As seen in equation (20) above, the known (or presumably known)constants include the speed of light c, the direction cosines k_(X),k_(T), k_(Z) for the vector from node A to node P, the position (A_(X),A_(Y), A_(Z)) of node A, the position (P_(X), P_(Y), P_(Z)) of node P,and the constants m_(A), n_(A), q_(A), and r_(A). Thus, equation (20)has eight unknowns, namely, location solutions LS1 _(X), LS1 _(Y), LS1_(Z), LS2 _(X), LS2 _(Y), LS2 _(Z) and clock solutions CS1, CS2. Eachtime that another node is added to the network such that node P receivesa ping transmit event with the additional node's current space-timestate, node P creates an additional equation with these same unknowns.Thus, for a network with more than eight nodes in communication with oneanother, any node within the network is able to solve for the eightunknowns in equation (20) to determine its own location solutions andclock solutions.

Thus, by using ping transmit events received from a sufficient number ofother nodes in the network 800, the enhanced PhaseNet algorithmsdisclosed herein allow each node of the network to solve for its ownclock drift and location. In addition to periodically determining theclock solutions CS1, CS2 and the location solutions LS1, LS2, each nodealso broadcasts ping transmit events with its current space-time state.In general, a node may need to broadcast its space-time state at a timelater than that of the latest solution which it has calculated. This canbe dealt with through standard extrapolation methods applied to theprior solutions. With reference to FIG. 9, for example, node P mayreceive a sufficient number of ping transmit events during a solutionperiod between 1000 counts and 2000 counts to solve for the clocksolutions CS1, CS2 and location solutions LS1, LS2, and may transmit itsown current space-time state upon reaching 3000 clock counts. In such anembodiment, node P may use the clock count C_(CS2) (e.g., as itstransmit count C_(tP)) that has been adjusted based on the solved clocksolution CS2. However, equation (20) above does not provide a locationsolution LS3 corresponding to this clock solution CS2 at 3000 counts.Thus, node P may extrapolate the location solution LS3 using knownextrapolation techniques based on the location solutions LS1, LS2 (andany previously determined location solutions).

FIG. 10 is a flowchart of a method 1000 for determining location andtiming information of a local node P in a network according to oneembodiment. The method 1000 includes receiving 1010 a ping transmitevent with node A's current space-time state. As discussed above, nodeA's current space-time state includes a transmit count stamp C_(tA)corresponding to a time T_(tA) when node A transmitted the particularping transmit event received by node P. Thus, the transmit count stampC_(tA) is based on a remote clock operating within node A. Node A'scurrent space-time state also includes a location (A_(Z), A_(Y), A_(Z))of node A at the transmit time T_(tA).

After receiving the ping transmit event with node A's current space-timestate, the method 1000 includes producing a ping receive event, that isto say, associating 1012 the received ping transmit event with a receivecount stamp C_(rPA). The receive count stamp C_(rPA) corresponds to alocal time, based on an independent local clock within node P, when theping transmit event is received. The method 1000 also includesassociating 1014 the received ping transmit event with node P's (local)location (P_(X), P_(Y), P_(Z)) at the local time when the ping transmitevent is received. As discussed above, the receive count stamp C_(rPA)and the local location (P_(X), P_(Y), P_(Z)) may be estimates based onnode P's last known location and time.

The method 1000 also includes generating 1016 a linear equation relatinga first clock solution CS1, a second clock solution CS2, a firstlocation solution LS1 (LS1 _(X), LS1 _(Y), LS1 _(Z)), and a secondlocation solution LS2 (LS2 _(X), LS2 _(Y), LS2 _(Z)) to node A's currentspace-time state [C_(tA), (A_(X), A_(Y), A_(Z))], the receive countstamp C_(rPA), and the node P's position (P_(X), P_(Y), P_(Z)). Asdiscussed above, the equation generated corresponds to equation (20) andthe location solutions LS1 and LS2 include X, Y, and Z components. Thisprovides a single equation with eight unknowns. The method 1000 thenqueries 1018 whether there are more ping transmit events received fromother nodes during the solution period that need to be processed. Withreference to FIG. 9, for example, the solution period corresponds to apredetermined number of local clock counts (e.g., 1000 clock counts) onnode P between the first location solution clock count C_(LS1) and thesecond location solution clock count C_(LS2). As discussed above, atleast eight ping transmit events are needed to create eight equations tosolve for the eight unknowns. Thus, the method 1000 may cycle throughsteps 1010, 1012, 1014, and 1016 at least eight times. Although notshown in FIG. 10, in some embodiments, the method 1000 also determinesthat an insufficient number of ping transmit events have been receivedduring the solution period. In such embodiments, the method 1000 maynotify the system and/or a user that node P cannot determine itsspace-time solution for the particular solution period or that node P'sspace-time solution for the particular solution period is based on“unreliable information.”

For illustrative purposes, the notation shown in FIG. 10 corresponds tonode P receiving a ping transmit event from node A. An artisan willunderstand from the disclosure herein, however, that the method 1000includes receiving ping transmit events from a plurality of differentnodes and that the notation shown in FIG. 10 may be adjustedaccordingly. For example, node P may also receive 1010 a ping transmitevent with node B's current space-time state [C_(tB), (B_(X), B_(Y),B_(Z))]. In such a case, node P produces a ping receive event in whichit associates 1012 the received ping transmit event from node B with areceive count stamp C_(rPB), and eventually generates an equation in theform of equation (20) with the following notation:

$\begin{matrix}{{( {C_{rPB} - C_{tB}} ) - \frac{\sqrt{( {B_{X} - P_{X}} )^{2} + ( {B_{Y} - P_{Y}} )^{2} + ( {B_{Z} - P_{Z}} )^{2}}}{c}} = {{k_{X}\lbrack {{m_{B}( {{LS}\; 1_{X}} )} + {n_{B}( {{LS}\; 2_{X}} )}} \rbrack} + {k_{Y}\lbrack {{m_{B}( {{LS}\; 1_{Y}} )} + {n_{B}( {{LS}\; 2_{Y}} )}} \rbrack} + {k_{Z}\lbrack {{m_{B}( {{LS}\; 1_{Z}} )} + {n_{B}( {{LS}\; 2_{Z}} )}} \rbrack} - {\lbrack {{q_{B}( {{CS}\; 1} )} + {r_{B}( {{CS}\; 2} )}} \rbrack.}}} & (25)\end{matrix}$

An artisan will recognize that equation (25) includes the same unknownsas equation (20) above, namely, location solutions LS1 _(X), LS1 _(Y),LS1 _(Z), LS2 _(X), LS2 _(Y), LS2 _(Z) and clock solutions CS1, CS2.Accordingly, the matrix equation (19) discussed above grows as pingtransmit events are received from more nodes. Once node P receives pingtransmit events from eight different nodes (e.g., from at least nodes A,B, C, D, E, F, G, and H), the method 1000 solves 1020 the system oflinear equations (e.g., using known matrix algebra techniques) for theseunknowns.

The method 1000 then uses 1022 the location solutions LS1, LS2 toestimate node P's current location (P_(X), P_(Y), P_(Z)). The method1000 also uses 1024 the clock solutions CS1, CS2 to correct node P'slocal clock with respect to a network system time. Node P may also usethe location solutions LS1, LS2 and the clock solutions CS1, CS2 tocorrect the space-time solutions for all of the nodes from which itreceived a ping transmit event. Stated another way, node P may solve thesingle matrix equation (19) to determine its own space-time solution andthe space time solutions of all the nodes in communication with node P.As a service back to the network, node P then transmits 1026 its currentspace-time state [C_(tP), (P_(X), P_(Y), P_(Z))] to the other nodes,where C_(tP) is node P's corrected clock count based on the clocksolutions CS1, CS2 and (P_(X), P_(Y), P_(Z)) is node P's estimatedposition based on the location solutions LS1, LS2. An artisan willrecognize from the disclosure herein that the method 1000 shown in FIG.10 may repeat itself periodically whenever a location solution LS and/ora clock solution CS is desired. In the embodiment of FIG. 9, forexample, the method 1000 may be repeated every 2000 counts so as toobtain at least two clock solutions CS1, CS2 and two location solutionsLS1, LS2 during each time the method 1000 is performed.

Example ADS-B Embodiment

As discussed above, in certain embodiments, the enhanced PhaseNettechnology disclosed herein is used as a backup to other timing/locationsystems and methods that provide initial space-time states to the nodes.In one example aviation embodiment, the enhanced PhaseNet technologydisclosed herein is used as backup for an ADS-B system to reducevulnerability to GPS outages.

The Federal Aviation Administration's (FAA's) “NextGen” approach tonavigation and air traffic control uses ADS-B and several other emergingtechnologies in a comprehensive approach to improve capacity, safety,security, environmental impact of aviation, and overall efficiency.While at one level this represents significant change to variouscomponents of the airspace system, it is also an evolution of knownpractices and technology. This includes an increased reliance on GPS,digital databases, visualization technology, and automation tools. Thus,there is a need to reduce the system's vulnerability to GPS outages.

ADS-B promises to provide improvements to aviation for the foreseeablefuture. Initially, ADS-B was linked to improving safety. The improvedperformance has also been quantified to provide required navigationperformance (RNP), which allows increased capacity and efficiency. Thisflexibility has led to the FAA decision to accelerate implementation ofADS-B throughout the National Airspace System (NAS). Other countries,including China, have also decided to adopt ADS-B to improve theiraviation enterprise. RNP requirements seek to allow a variety oftechnologies to comply with the specifications to facilitate futureinnovation and implementation of new technology and do not dictate ADS-Bor GPS use. However, as a practical matter, the vast majority of themost stringent RNP requirements are routinely met using some combinationof GPS or other SATNAV augmentations. Today, capacity and safety aregenerally the focus of RNP. In the future, efficiency in the form offuel savings and environmental friendliness in the form of lower carbonemissions are likely to be at least as important. The PhaseNetembodiments disclosed herein provide an innovative, software-basedsolution to enhance the ADS-B architecture by providing improved timingand location for a full-up system and a cost effective SATNAV backupthat is independent of GNSS.

FIG. 11 schematically illustrates nodes (ADS-B nodes) in an ADS-B system1100 according to one embodiment. The ADS-B nodes may be located, forexample, in aircraft 1110, 1112, 1114, 1116 that may be in the air or inthe process of landing or taking off, aircraft 1118 that may be taxiing,aircraft 1120 that may parked at an airport terminal 1122, airportcontrol towers 1124, fixed communication systems or navigation systems1126, or other devices such as airport ground vehicles 1128, 1130. EachADS-B node is configured to communicate (as depicted by the concentriccircles illustrated in dashed lines) with one or more of the other ADS-Bnodes. An artisan will recognize from the disclosure herein that manyother vehicles or fixed devices may also communicate with one or more ofthe ADS-B nodes illustrated in FIG. 11, and thus become an ADS-B nodeitself.

In certain embodiments, the amount of data available for transmittingspace-time information between ADS-B nodes is limited. Thus, thisexample uses the enhanced PhaseNet algorithms discussed above to reducenetwork traffic. An artisan will recognize from the disclosure herein,however, that the general PhaseNet algorithms (e.g., using both pingsand pungs) may also be used in certain embodiments.

During normal operation, the ADS-B system 1100 uses GPS to providetiming and location information to aircraft 1110, 1112, 1114, 1116. WhenGPS becomes unavailable, however, each ADS-B node according to oneembodiment determines its initial space-time state based on its lastknown time and location (e.g., as provided by the GPS before theoutage). Each ADS-B node then transmits its initial space-time state asa ping transmit event and receives ping transmit events with space-timeinformation from other ADS-B nodes. As discussed in detail above, eachADS-B node then uses its initial space-time state and the space-timestates in the ping transmit events received from the other ADS-B nodesto update its own space-time solution. Thus, the embodiments disclosedherein allow ADS-B nodes to “ride through” periods of GPS outage withminimal degradation of system accuracy. In comparison to other backupsystems, the embodiments disclosed herein provide at least the followingadvantages:

-   -   The embodiments discussed herein do not require new avionics in        addition to that required by ADS-B, which greatly simplifies the        aircraft components of the system.    -   The embodiments discussed herein do not require a separate        ground infrastructure in addition to that required for ADS-B.    -   The embodiments discussed herein are independent of, and are of        a significantly different mode of operation than, GNSS.    -   The embodiments discussed herein are globally extensible.    -   The embodiments discussed herein are comparably low cost because        they highly leverage the ADS-B system.

The embodiments discussed herein may be deployed in ADS-B systems withlow impact on communications and processing requirements. The existingmedia access control (MAC) layer used in the universal accesstransceiver (UAT) utilizes statistical time division multiplexing, withno retransmission due to collisions. This protocol is well suited to thegeneral and enhanced PhaseNet algorithms discussed herein and ensureshigh probability of regular data traffic between nodes. To allowvalidation of ADS-B reported data, the UAT provides time stamping ofboth transmitted and received packets. Thus, these same time stamps maybe used with the embodiments discussed herein so that no hardwaremodifications to the UAT are required to provide time stamping ofmessages. In certain embodiments, minimal changes to the ADS-B system toincorporate the PhaseNet algorithms discussed herein include dedicatinga modest number of additional bits to PhaseNet pung data (for generalPhaseNet algorithms).

Further, because the PhaseNet algorithms discussed herein may beimplemented separately by each ADS-B node, there is no centralizedsingle point of failure of a PhaseNet-enabled system. Instead, thefailure of any single component of the augmented ADS-B/PhaseNet systemresults in a minimal loss of overall system performance.

PhaseNet technology is well suited for current and developingtechnologies. Modern digital communication systems allow time stampingof messages to be accomplished with a negligible increase in radiocomplexity. Further, the computing power required to implement PhaseNetalgorithms is readily available in modern cockpits. In addition, therehas been a many orders of magnitude improvement in storage capacityrelative to capacity, size, and power consumption. These trends havecome so far so fast that smart phones have an abundance of all threeenabling technologies, (DSP, processing, and storage) compared, forexample, to the technology used the last time men walked on the moon.

Thus, PhaseNet technology is in complete contrast to the other proposedSATNAV backups, most of which were operational during World War II.These include radar, long range aid to navigation (LORAN), distancemeasuring equipment (DME), and inertial navigation systems (INS). FIG.12 illustrates a table that compares limitations and features of variousSATNAV backup systems under consideration for aviation. In the tableshown in FIG. 12 a “multilateration” system is an approach whereaircraft periodically send signals that are received at several groundlocations that determine aircraft position by the time difference ofarrival principle; calculated positions are then transmitted back to theaircraft. Multilateration is generally the same as direction finding(DF) used since the dawn of aviation as a primary and backup radionavigation technique.

In summary of this example embodiment, GPS has become a global utilityand the aviation community has adopted the capabilities made availablefrom SATNAV services. In aviation, however, there are high consequencesof GPS/GNSS unreliability which can be catastrophic. Long term satellitenavigation outages may incur wide-spread economic paralysis without aviable infrastructure component to provide reliable aviation servicesindependent of GPS. This has led to a consensus that some sort ofsatellite navigation backup should be part of the NextGen airtransportation system. Nearly all the proposed solutions to providebackup are vintage systems that do not take technological advances andare looking at the world through the rear view mirror. They have beenabandoned primarily because they are expensive to maintain and provideinadequate accuracy. The PhaseNet embodiments discussed herein haveseveral very beneficial qualities that can cost effectively provide arobust backup capability. The PhaseNet embodiments discussed herein alsohave the potential to work with a full-up GPS/GNSS system to provideadditional accuracy and precision that can enable better, fuel savingroutes and minimize carbon emissions in a way that looks forward to abetter aviation enterprise ahead.

It will be understood to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

1. A method of determining location and timing information of a localnode in a network, the method comprising: receiving pings from aplurality of nodes in the network, each ping comprising: a transmitcount stamp corresponding to a remote time when the respective nodetransmits the ping, the remote time based on an independent remote clockoperating in a respective node; and a remote location of the respectivenode at the remote time when the respective node transmits the ping; foreach ping received within a predetermined time interval: associating areceive count stamp with the received ping, the receive count stampcorresponding to a local time when the ping is received, the local timebased on the independent local clock; and associating a local locationwith the ping, the local location corresponding to the local time whenthe ping is received; and processing the transmit count stamps, theremote locations, the receive count stamps, and the local locationscorresponding to the pings received within the predetermined timeinterval to determine one or more clock solutions for correlating theindependent local clock with a network time, and to determine one ormore location solutions for estimating a new local location.
 2. Themethod of claim 1, further comprising: processing the one or more clocksolutions and the one or more location solutions to determine a localspace-time state comprising: a local count stamp corresponding to alocal time when the local space-time state is transmitted, the localtime based on the independent local clock as correlated with the networktime by the one or more clock solutions; and a local locationcorresponding to when local space-time state is transmitted, thetransmitted local location estimated using the one or more locationsolutions and the local count stamp.
 3. The method of claim 2, furthercomprising: transmitting the local space-time state to the plurality ofnodes in the network.
 4. The method of claim 1, further comprising:providing an initial local space-time state based on data received froma timing and/or location system.
 5. The method of claim 4, furthercomprising: transmitting the initial local space-time state to theplurality of nodes in the network.
 6. The method of claim 4, furthercomprising: determining the receive count stamps associated with eachping received during an initial solution period based on the initiallocal space-time state.
 7. The method of claim 4, further comprising:determining the local location associated with each ping received duringan initial solution period based on the initial local space-time state.8. The method of claim 4, further comprising: determining an initialdirection vector between a remote node and a local node based on thedata received from the timing and/or location system.
 9. The method ofclaim 4, wherein the timing and/or location system comprises a satellitebased global positioning system (GPS).
 10. The method of claim 4,wherein the timing and/or location system comprises an AutomaticDependent Surveillance-Broadcast (ADS-B) system, and wherein at leastone of the plurality of nodes is located within an aircraft, the methodfurther comprising: determining that satellite based navigation used bythe ADS-B system is unavailable; and in response to the unavailabilityof the satellite based navigation, determining the one or more clocksolutions and the one or more location solutions.
 11. A system fordetermining location and timing information, the system comprising: aplurality of nodes in communication with one another, each nodecomprising: an independent clock for generating count stamps; acommunication device for receiving remote pings from the other nodes,each remote ping including a transmit count stamp and a remote locationof a transmitting node; and a processor for: associating each remoteping with a receive count stamp based on the independent clock;associating each remote ping with a local location; and processing thetransmit count stamps, the remote locations, the receive count stamps,and the local locations to determine one or more clock solutions forcorrelating the independent clock with a network time, and to determineone or more location solutions for estimating a new local location. 12.The system of claim 11, wherein the processor of each node is furtherconfigured to process the one or more clock solutions and the one ormore location solutions to determine a local space-time state, andwherein the communication device of each node is further configured totransmit its local space-time state to the other nodes in the network.13. The system of claim 11, wherein the communication device of eachnode is further configured to receive an initial local space-time statebased on data received from a timing and/or location system and totransmit the initial local space-time state to the other nodes, andwherein the processor of each node is further configured to determinethe receive count stamps and the local locations associated with eachreceived remote ping based on the initial local space-time state. 14.The system of claim 13, wherein the timing and/or location systemcomprises a satellite based global positioning system (GPS).
 15. Thesystem of claim 13, wherein the timing and/or location system comprisesan Automatic Dependent Surveillance-Broadcast (ADS-B) system, wherein atleast one of the plurality of nodes is located within an aircraft, andwherein the processor of each node is further configured to: determinethat satellite based navigation used by the ADS-B system is unavailable;and in response to the unavailability of the satellite based navigation,determine the one or more clock solutions and the one or more locationsolutions.